The invention relates to a method and device for controlling register settings of a printing press, as well as to a printing press provided with such a device.
For multi-color printing, at least two color components are printed over each other in a printing press with the aid of a plurality of imaging cylinders. If a printing press is in register or, differently expressed, is in an overlapping fit, the images printed by the different print units, which respectively print on a single color component, are placed to fit precisely one above the other.
In web-fed presses, a print medium or substrate, for example, paper or film, frequently travels long distances between two print units. Devices are integrated into these web-fed presses which make it possible to precisely adjust the register settings and to continuously re-adjust the register settings during the course of the printing operation. Such devices are also used in sheet-fed printing presses.
Markings such as the so-called registration marks or registration crosses are auxiliary means used for known register settings. These markings are also printed on outside of the picture area by each printing press. With the aid of image-processing systems, the positions of the respectively relevant registration marks, relative to each other, are recorded as well as evaluated and the printing press is readjusted accordingly.
To evaluate the register or the register accuracy, a distinction is made between the lineal register (in the movement direction of the web of print medium), the side or lateral register (transverse to the movement direction) and the diagonal register. The problem defined for the invention essentially relates to the lineal register.
For example, deviations detected in the lineal register can be compensated for by displacing individual guide rollers, meaning the register rollers, which shorten or lengthen the distance between two print units. Alternatively, the lineal register can also be adjusted by rotating the imaging cylinders relative to each other.
A deviation in the register is normally caused by the use of a different type of print medium which exhibits a different expansion behavior as compared to the previously used print medium. The speed of the printing press can also require a readjustment of the register because the print medium web behaves differently, depending on the speed of the printing press.
Controls for register settings of this type are disclosed, for example, in the German patent documents DE 102006060212 A1 or DE 10254836 A1.
Known register settings take into account, insufficiently or not, that the length of the print medium web section between the print units that take part in the printing operation can vary noticeably as a result of changes in the configuration of the printing press. The deviations in the register can consequently exceed those normally occurring in the detection range for standard register settings, thus making it impossible for the register to be adapted automatically. Also not addressed as a subject is the influencing of the register as a result of a change specified by the machine operator in the length of the intended print image. With known printing presses, this is realized by specifying a heterodyne speed for the individual drives of the imaging cylinders belonging to adjacent print units, which participate in the image-printing process.
The length of the print image can be shortened by approximately up to 1 mm, relative to the original format of the print image, wherein this can be achieved through accelerating the drive of the imaging cylinder. Owing to the higher but constant rotational speed, as compared to the theoretically correct speed for the print medium, the print image is correspondingly shortened.
In contrast, the print image is lengthened by approximately up to 3 mm, relative to the original format for the print image, if the speed of the respective imaging cylinder drive is slowed as compared to the speed of the print medium web. Thus, the print image can be pulled over a longer section of the web than is provided based on a corresponding negative on the imaging cylinder.
Register deviations caused by changes in the length of the print image as a rule do not fall within the detection range of the above-discussed register settings. For example, if five print images and/or formats, which must respectively be lengthened, are located between two adjacent print units, the change in the length of each print image on the whole results in a register deviation in the downstream arranged print unit, as seen in production direction, in the order of magnitude of five times the change in length for the print image.
With printing presses that are especially flexible for use, the length of the print medium section between two adjacent print units frequently varies considerably as a result of production changes, so that it is not automatically certain just how many print images are located between two adjacent print units.